Convergent coupling of helium to the H/D background in magnetically confined plasmas

The alpha particles are coupled to the hydrogen/deuterium background plasma via charge exchange processes. Population kinetics calculations show that in the framework of standard models, the charge transfer from hydrogen/deuterium excited states to helium leads to a strong divergence of the atomic populations and to a critical dependence on the number of quantum states retained in the simulations. The divergence is related with the large cross-sections for charge transfer from excited states of the background atoms. For typical parameter conditions (e.g., ITER) the charge-exchange-induced population flow may even reach the level of flow typically induced by turbulent diffusion. A self-consistent excited-states coupling approach of atomic kinetics is proposed, which avoids divergences and critical dependence on the number of states. This approach also removes free parameters from the system and provides therefore the ideal basis for a pure diffusion analysis in space which is requested to test global effects of turbulent transport theories.